2005-2007 Ford Mustang GT-Big-Bore Twin-Screw Supercharger Kit

Making boost from a supercharger is a game of give and take. To generate boost, it requires some engine power to spin the supercharger's impellers. Simply put, higher boost levels will take additional engine power to spin the supercharger faster. Add in the fact that compressing air will also heat it substantially, and it's easy to see how you can quickly reach a point of diminishing returns.

How do you break the cycle? You can start with a more efficient blower, for one thing. Kenne Bell decided to start with a proprietary design twin-screw supercharger, which is (according to them) more efficient at compressing air. The advantage of the twin-screw's efficient design is that it can make more power with less engine rpm at lower boost levels. Compared to the flat or slightly angled impellers of a standard Roots-style supercharger, the twin-screw's impellers have high blade angles and are set at extremely close tolerances: 0.003-0.004 inch. The blades look like two screws that spin together to take in larger amounts of air and force it into the engine. The tight blade gap allows the twin-screw to run frictionless, which dramatically reduces heat and keeps the discharge temperature of the air much cooler.

While other Roots-style superchargers have recently moved to higher impeller angles that lead to a more efficient unit, Kenne Bell jumped ahead in the evolution of the supercharger to further improve on its efficiency. The next step was to go big. Through extensive dyno testing of iterative designs, it was evident that larger supercharger housings would be able to handle higher volumes of air. Furthermore, improvements in impeller bearings and shaft designs would mean that it would require less horsepower to spin.

For comparison purposes, the latest Roots-style superchargers use 2.1 and 2.3L cases. Kenne Bell's newest twin-screw designs use 2.6 and 2.8L cases. According to Jim Bell of Kenne Bell, the new twin-screw 2.6 and 2.8L designs require up to 56 hp less to drive. Additionally, they operate about 30 degrees cooler than the previous model twin-screw superchargers. These are substantial improvements that are already making a big hit with many enthusiasts swapping out to bigger twin-screw superchargers in their factory Ford Lightning pickups, Ford GTs, and Mustang GT500s.

Kenne Bell tells us its 2.8L supercharger is the largest 2L series in the industry, and they are very popular with late-model Z06 Corvette and Mustang GT500 owners. Considering that the supercharger can reportedly make anywhere from 700-800 hp on these vehicles with a stock engine, you can see that supercharger efficiency makes a huge difference.

The Big-Bore 2.6L supercharger is one of Kenne Bell's most popular for the standard 4.6L Ford Mustang GT. When compared to other Roots-style Mustang blower kits that use a 2.3 or 1.9L housing, the Kenne Bell 2.6L Big-Bore offers greater power potential with more available air. By bolting one on a stock '07 Mustang GT with no other modifications, you can transform an engine that's rated at 300 hp into one that can make more than 500 hp at the crank. According to Bell, at 5 psi of boost, the Big-Bore 2.6L twin-screw supercharger makes 401 rear-wheel horsepower. At 10 psi of boost, the kit can generate 500 rear-wheel horsepower. That's about 600 hp at the crank from a stock 4.6L Ford V-8 engine.

These figures are difficult to believe, and might be partly explained by the generous Dynojet chassis dyno employed, but we had to find out for ourselves if the 2.6L supercharger would match the company's claims. The kit comes with a high-flow aluminum intake manifold that replaces the factory plastic intake. If you order the intercooled version, the intercooler is set inside this new intake and includes a fluid pump, reservoir, and heat exchanger. In order to take full advantage of the higher air volume entering the engine, Kenne Bell also supplies a 90mm mass air meter and a high-flow cold air intake kit to ensure that the supercharger receives the coldest charge.

ECU modifications are done with a handheld programmer, and the factory fuel injectors are upgraded to a set of 39 lb/hr units. Furthermore, to ensure that there's enough fuel pressure getting to the engine, Kenne Bell's Boost-A-Pump adds higher voltage to the factory fuel pump under boost to ensure the air/fuel mixture never goes lean. This combination of the injector and Boost-A-Pump can support 525 rear-wheel horsepower.

The Kenne Bell Mustang kit is very detailed, and comes with a 200-plus-page installation booklet that is filled with step-by-step instructions and large images. Moreover, the kit comes with a mix of Kenne Bell-designed parts, as well as factory Ford replacement gaskets and components. Kenne Bell also provides a T-handle hex wrench and a fuel line removal tool that is required to safely unlock and remove the factory lines.

The installation takes a couple of days for a qualified installer (approximately 14 hours), and is extensive. Nevertheless, it can be done by someone with experience working on late-model Mustangs. Most of the time is spent taking care of the wiring required to extend the harness in key areas. This includes extending the new MAF sensor wiring and attaching the Boost-A-Pump to the factory harness in the vehicle's trunk.

The front fascia must also be removed to install the heat exchanger, which bolts behind the factory bumper. Since the factory throttle body is relocated to the passenger side of the engine, a new air intake duct is routed through the radiator core support, which requires some trimming. It attaches to a cotton/gauze air filter and MAF sensor, which gets located under the passenger-side headlight. According to Bell, this is the area where the coolest air is available, and it is completely shielded from radiator and engine heat.

Some of the factory cooling lines that are routed under the factory intake manifold are slightly modified to make room for the Kenne Bell intercooled manifold. The alternator also sits in a new bracket and new pulleys are added to accommodate a longer serpentine belt that wraps around the supercharger's impeller pulley.

Once the manifold and supercharger are bolted in, the engine begins to take on a more menacing appearance. The factory injector rails are retained, but with the new 39 lb/hr injectors that are also supplied in the kit. In fact, there isn't very much that needs to be modified to install this system. According to Bell, everything is designed so that the engine can be put back to stock if you desire.

With the Big-Bore kit installed, the Mustang was placed on the Dynojet chassis dyno. Prior to the installation, the baseline run was done on a completely stock '07 GT. The baseline horsepower levels were consistent with other '05-07 Mustang GTs we've tested, making a peak of 272 rear-wheel horsepower and 296 lb-ft of torque. (You'll remember from our July 2006 issue that our previous '05 Mustang GT five-speed tagged the Dynojet's meter at 264 peak horsepower, and a prior '05 GT from the Ford press fleet managed 266 on the same dyno.-Ed.) At the 5-psi level, the Mustang netted a peak of 401 rear-wheel horsepower and 357 lb-ft of torque. A very impressive result for a minimal amount of boost.

The pulley was then changed to the 3.75-inch unit to produce 9-10 pounds of boost, and the Mustang responded with a peak of 482 rear-wheel horsepower. According to Bell, this horsepower level is the limit of the engine, and it would require additional internal engine upgrades to go higher. Nevertheless, if you compare the horsepower figures of this supercharged 4.6L V-8 with the dyno numbers of a factory supercharged 5.4L GT500, this Mustang GT is making about 50 more horsepower at the wheels. In addition, the Big-Bore 2.6L supercharger is capable of 700-plus horsepower with forged pistons, a stronger clutch, a forged crank, and beefier connecting rods.

If you're envious of the fact that this kind of power is only available for Ford Mustang owners, rest assured that Kenne Bell's large-displacement supercharger kits are available for GM LS1, LS2, and LS6 engines, as well as earlier-model Ford 5.0L and 4.6L engines. Any of these engines make a great platform for vintage musclecars and street rods. With the addition of a Kenne Bell 2.6 or 2.8L supercharger kit, the horsepower level can easily reach 700-1,000 if you're willing to go big.